A number of chemical analytical techniques utilize columns for detecting or measuring an analyte of interest. The columns each include a cylindrical tube of a particular length and inner diameter dictated by experimental requirements that are filled with selectively adsorbent packing materials. An analyte or mixture of analytes (the “sample”) dissolved in a solution (the “sample matrix”) is introduced at one end of the column, and then a carrier fluid is run through the column. The carrier fluid brings the sample matrix along with it.
As analytes travel through and around the column packing material, the analytes interact with the column packing material to varying degrees according to the analytes' chemical affinity for the packing material. The greater the affinity of a particular analyte for the packing material, the longer it will take for that analyte to travel the length of the column. Analytes that have no affinity whatsoever for the packing material will travel at approximately the same speed as the carrier solvent, while analytes with affinity for the packing material will be delayed by an amount generally proportional to that affinity. Therefore, a single analyte in solution can be separated from its sample matrix, or a mixture of analytes in solution can be separated both from the sample matrix and from each other, based on differing affinities for a given packing material. Such techniques are used in liquid chromatography (LC) as well as in situations where LC is combined with other instrumentation (liquid chromatography-mass spectrometry, or LCMS, for example).
Most packing materials include either regular (spherical) or irregular particles, with a predetermined nominal diameter. Actual particle diameters are likely to be within a normal distribution around this predetermined nominal diameter. A design requirement of chromatography column hardware is that the hardware must allow liquids to pass into and out of the column, while keeping the packing material immobilized within the column tube. This is often accomplished by use of a porous substance called a “frit” disposed at an inlet end and at an outlet end of the column. This frit has a rated porosity that is smaller than that of the smallest expected packing material particles.
The outlet frit of a column is more than just a barrier for keeping packing material in place. The frit actually plays an important role in determining overall column performance. Packing materials, particularly those that are formed from regular, spherical particles, must be placed into a column in a way that ensures the packing material is tightly packed and evenly distributed, without voids, channels, and other irregularities. Any deviance from a perfectly packed bed will reduce the effective separating power and performance of the analytical column. Columns are packed by sending packing material slurry through the column, which is open at the inlet. The slurry solvent passes through the outlet frit, while the packing material collects at the frit surface, gradually filling the column. And so the outlet frit is actually the foundation upon which the packed bed is built. Thus, the method used to retain the frit at the column outlet is preferably mechanically durable. It is also preferable that the seal between the frit and the column be as close to hermetic as practically possible. This ensures that the only possible flow path out of the column is through the frit. Internal volumes should also be kept as low as possible to minimize “mixing” effects, which can serve to decrease instrument sensitivity and response.
Previously developed columns are manufactured with one of a few methods for keeping outlet frits in place. A frit can be placed at the surface of a tube and secured via an external compression fitting. Or, the frit can be placed within the tube diameter and secured there. This second approach is desirable in terms of keeping internal volumes to a minimum.
Current methods employed in securing frits within the inner diameter of column tubes include:                Interference fit, where the diameter of the frit is selected to be slightly larger by a precise amount than the inner diameter of the tubing, and the frit is forcibly pressed into the smaller cavity, resulting in a friction fit;        Adhesive bonding, where a chemical adhesive is used to provide a bond between the frit and the inner tube wall;        Staking, either a roll-stake or orbital stake method, where the frit is placed within a counter-bored cavity with a thin wall at the end of the tube, and this thin walled material is then rolled over the side and front edge of the frit;        Sintering, where the frit is actually produced in situ within the tube end, rather than being manufactured separately;        
Controlled atmosphere brazing, where the frit is brazed onto the end of tube in a controlled atmosphere; and                Welding, where, referring to FIG. 1, a frit 12 is welded onto a distal end of a tube 14 to form a column 10. This is accomplished by inserting a porous frit 12 into a recess 16 disposed in a distal end of the tube 14. A ring of solder 18, such as silver, is placed along an upper edge of an annular space 20 disposed between the frit 12 and the tube 14. The column 10 is placed in an inert environment and heated, such as by placing the column 10 in an oven, to cause the solder 18 to melt. The melted solder 18 flows in the annular space 20 as shown in FIG. 2. As the solder 18 cools, the outer surface of the frit 12 is bonded to the inner surface of the tube 14 by the solder 18.        
The current trend in column hardware technology is toward smaller bed volumes. It is typical for a given sample to be present in very low amounts, or in very low concentrations. Keeping internal volume to an absolute minimum is necessary to avoid dilution of the sample during analysis. Some of the above methods of frit retention are not amenable to use in low-volume applications, while others have limitations and drawbacks of a different kind. Problems of previously developed frit coupling techniques include:                Sample contamination potential from the adhesives used to adhere the frit to the tube;        Non-hermetic seal formed when the frit is attached using staking and interference fit techniques;        Residue left within the tube when in situ sintering techniques are used; and        Referring to FIG. 2, when welding frits 12 to tubes 14 using previously developed welding techniques, the welding must occur in an inert environment to prevent welding residues from contaminating the column 10, thereby increasing the difficulty and expense of welding the frit 12 to the tube 14. Further, it has been found that previous welding techniques are imprecise and unsuitable for frit 12 diameters less than about 0.25 of an inch since the flow of the solder 18 cannot be accurately controlled and may flow into a central bore 22 of the tube 14 interfering with a flow of a sample through the column 10. Further, previously developed welding techniques do not substantially reduce the porous volume of the frit 12 since the solder 18 only adheres to the outer surface of the frit 12 and does not fill in more than a negligible amount, if any, of the pores of the frit 12. Thus, the relatively large porous volume of the frit 12 absorbs a portion of the sample passing through the column 10 creating a dead space in a column 10, thereby introducing error into the testing process and increasing the amount of sample needed to perform the test.        
Thus, there exists a need for a column having a frit attached to a tube of the column that is reliable, relatively inexpensive, reduces testing error, and which does not contaminate a sample passing through the column.